Regulator or resistor (newbie)

[LeonR]

Hi,

My robot is slowly building up and im learning alot about electronics by doing it (which is a good thing).

I've made a circuit for powering the picaxe board from an AC adapter using the example within the AXE020 manual to save on batteries. It uses a regulator which seems to do the job fine.

Anyway, soon my robot will require a second power supply connected to the project board to power the motors and a servo etc etc.

I would like to try and keep everything on V2 (apart from the microcontroller). Is it ok to maybe use a 12v rechargable battery (the sort model air craft might use), then connect that to my project board, but use a regulator between some components?

For example... 12v to motors....but then have a regulator to limit the power to 4.5-5v for a range sensor etc?

Reason I ask is that the regulators get very hot, and this seems like a big waste of energy from batteries which you want to last as long as possible.
I've seen various batteries about (rechargable) which are around 12v 1200mah , which I assume means it can provide 1200ma for 1 hour @ 12v? (or did i misunderstand this?).

Would using resistors be a better approch as the input on v2 is a known voltage?

Another question I have is about amps... if you have 12v at 1A (for example) and a device only requires 10ma.... it will be fine I assume? Aslong as the V is correct?

Im sure these are very basic questions in the world of electronics!

Thanks!!

 

[moxhamj]

If you are using 5V motors, one solution is to use 4 nicads. 4x1.2=4.8V. This is what the radio control guys use for planes etc - 4 nicads is a very standard system.

Your motors draw amps, so you want to match the volts from the batteries so you don't waste power. Heat = wasted power.

But for the electronics, this only draws milliamps, so it doesn't matter so much if you are regulating down from 12V to 5V. Running just a picaxe at 5mA - the reg won't even get warm.

Then there is the whole world of switched regulators - yes these convert voltages much more efficiently - but they are more complex and you can get stuck building the perfect switching reg and lose sight of the fun bit of building the robot.

It helps to keep the power supplies for motors and picaxes seperate. So you could have 4 nicads or NiMHs for the motors. And you could use 8 smaller nicads (eg AAAs) to run the picaxe via a 5V reg. Join the two supplies with a common ground (negative).

 

[westaust55]

I've seen various batteries about (rechargable) which are around 12v 1200mah , which I assume means it can provide 1200ma for 1 hour @ 12v? (or did i misunderstand this?).

Batteris are given a Ah (or mAh) rating based on the C5 discharge rate. The is based on the energy discharged over a 5 hour period.
So for the 1200mAh batteris the C5 discharge rate is 1200/5 = 240mA.

If you have a discharge current of 1200mA then you will get far less that the 1200mA - or in other words a lower effective capacity. For example, something like 900mAh.

If you look in the datasheet for the batteris you have you will find curves for capacity at different discharge rates.

 

[LeonR]

Thanks for the help guys, that helped me out quite a bit

As for the V2 battery voltage, my motors are actually lego motors (my robot is made of lego as its very easy to rebuild and design on the spot etc). The motors are meant to run at 9v. I've tried them at 5v from my projectboard supply and although they turn, the robot struggles to move properly. They draw about 120ma under load...

For anybody interested, this link is brilliant! Lego Motors


As the motors run at 9v , im thinking somthing like this?

And then regulate that down to ~5v for my other components, such as range finder(s), servo etc etc?

Thanks for the help again , I appreciate it. Without the forum community here I would struggle!

 

[Michael 2727]

Using a resistor will limit the current not the voltage, you need 2 resistors do do that.
Search voltage dividers. Putting a resistor before a regulator can work to reduce the heat/dissipation in a regulator but it is fraught with hidden dangers.
Resistors get hot which also wastes valuable power.

Your best bet is to match the motor supply as close to the motors as you can -
9.6V Supply 9V motors ( the battery will be going flat at 7.2V to 8V ) rather
than a 12V pack which may stress the motors and will produce a lot of wasted
heat/power.

Run the controller section from a smaller, say 4.8V pack and common both the NEGs
together as suggested.

Remembner most Standard Regulators require that the input side has around
2V to 2.5V higher voltage than the regulated output side to maintain correct
regulation, this is at all times and even under max load.
LDO Low Dropout regulators require much less (input side Voltage) voltage to
regulate the supply correctly, as low as 100mV, 0.1V

Speaking of Load it was good to see the Stall Currents included in the data for the
LEGGO motors.
This is very important for when your critter/vehicle zooms under the sofa and jams
up against the wall and it takes 3 minutes to finally get it out. It could be on fire by then.
More likely the regulator or something else has fried because people tend to take short
cuts using underated components.
Fuses are always a good idea.
They are cheaper than replacing a regulator or motor/servo etc.

 

[LeonR]

Ok ill make note of that info! thanks!

As for stall currents, yeh i was mainly interested in that. I checked the max current that the motor driver could handle, i think its somthing like 500ma per channel (I cant remember the exact number), but the stall current on my motor should be 360ma according to that website.

Im just assuming thats how it works using common sense, so shouldn't burn anything out with a bit of luck! I did gear down a faster motor to start with, but the stall current was higher than the motor driver limit, and im sure it will get stuck at some point as you say!


Thanks!!